A number of prominent spectral lines in the spectra of magnetic A and B main sequence stars are produced by closely spaced doublets or triplets. Depending on the strength and orientation of magnetic field, the PPB magnetic splitting can result in the Stokes I profiles of a spectral line that differ significantly from those predicted by the theory of Zeeman effect. Such lines should be treated using the theory of the partial Paschen-Back (PPB) effect. To estimate the error introduced by the use of the Zeeman approximation, numerical simulations have been performed for Si ii and Si iii lines assuming an oblique rotator model. The analysis indicates that for high precision studies of some spectral lines the PPB approach should be used if the field strength at the magnetic poles is Bp > 6-10 kG and V sin i < 15 km s−1. In the case of the Si ii line 5041 Å, the difference between the simulated PPB and Zeeman profiles is caused by a significant contribution from a so called “ghost” line. The Stokes I and V profiles of this particular line simulated in the PPB regime provide a significantly better fit to the observed profiles in the spectrum of the magnetic Ap star HD 318107 than the profiles calculated assuming the Zeeman effect.